Acrylic acid (prop-2-enoic acid) is a synthesis unit for the production of superabsorbents used principally in hygiene products, and a raw material for acrylate chemistry in its entirety. Acrylic acid is obtained on the industrial scale by catalytic vapour phase reaction of propene with oxygen. This gives rise not only to the desired acrylic acid, but also to undesired accompanying products which present difficulties in the further processing of the acrylic acid. These accompanying products are essentially acetic acid, aldehydes, dialdehydes, dicarboxylic acids and water. An important step in the preparation of acrylic acid (AA) is accordingly to purify the acrylic acid-containing composition originating from the vapour phase reaction to free it of the unwanted accompanying products, such that what remains is pure acrylic acid. The present invention addresses one aspect of this purification.
Acrylic acid is typically prepared by conventional chemical means via the oxidation of propene to acrolein and subsequent further oxidation to acrylic acid. EP 1319648 A1 discloses a process of this kind, in which acrylic acid (AA) is prepared by catalytic vapour phase oxidation of propylene in two steps. The process for preparing AA by the two-stage (the first catalytic oxidation reaction stage for the conversion of propylene principally to acrolein, and the second catalytic oxidation reaction stage for the conversion of acrolein to AA) catalytic vapour phase oxidation of propylene using molecular oxygen is already known and has been conducted on the industrial scale for many decades. For many reasons (inflammability limits for the propylene/air mixture and high heat of reaction), it is necessary to dilute the reaction gases with inert gases (for example water vapour, N2, CO2). A typical process for industrial preparation takes the following form: A mixture of propylene, air and steam is fed to a first oxidation reactor and the propylene is converted in the first step principally to acrolein and in small amounts to AA. The product is fed to a second oxidation reactor without separation. Fresh air and steam can, if required for the downstream oxidation reaction in the second oxidation step, be fed in at the inlet of the second oxidation reactor.
For the separation of gaseous AA from the steam in the output, which is at about 180° C., from the oxidation reactors, two separating processes are used: 1. Absorption of the gaseous AA in a high-boiling, hydrophobic, aromatic solvent at temperatures at which the process water remains in the process waste air gas, which leaves an absorption tower at the top (see, for example, DE 43 08 087 BASF, 15.09.94). 2. Absorption of AA in water with simultaneous quenching to low temperatures in a quench tower/collector in such a way that virtually all the process water evaporated condenses in the 180° C. output from the second reactor (see, for example, DE 30 42 468/Mitsubishi Chem. Corp. (MCC), 11.11.80). While the AA and high-boiling products are separated in many distillation steps in the first procedure, the water in the aqueous AA is removed in a subsequent azeotropic distillation after the quenching in the second procedure, in order to obtain a crude AA from which AA can be prepared with high purity for the preparation of AA copolymers or various AA esters.
In the second procedure, the handling of the process water, which consists of the water formed in the oxidation and the water for the dilution, which is required to be outside the inflammability limit, has a considerable influence on the economic viability of the process.
If the AA-containing product gas which is obtained at the outlet of the second oxidation reactor is introduced into the quench tower in order to obtain AA as an aqueous solution, the resulting process waste air gas containing unreacted propylene and other low-boiling organic materials leaves the quench tower at the top and has to be treated, for example, in an incineration furnace, such that no or virtually no organic compounds are emitted into the air (a flow process with one pass).
It is also possible that a portion of the process waste air gas is recycled and added to the propylene/air/steam stream at the inlet of the first reaction stage (cycle gas flow process), which is an entirely customary procedure particularly in the case that the process is operated with partial conversion of the propylene.
DE 10 2010 028 781 A1 describes the further separation of a mixture including aqueous acrylic acid obtained in a quench tower with the aid of a distillation column having a side draw.
U.S. Pat. No. 7,189,872 describes a process for preparing acrylic acid, in which a quench tower is used in order to separate acrylic acid from the gaseous product mixture. This aqueous mixture is subsequently separated by means of azeotropic distillation into a gaseous phase including water, and a liquid phase including acrylic acid.
In the acrylic acid preparation by oxidation of propene with atmospheric oxygen in low concentrations, secondary components are also formed, for example phthalic acid, terephthalic acid or isophthalic acid. These substances are high boiler compounds and are only sparingly soluble in water.
In the case of use of a quench tower for processing of the gaseous product mixture from the last oxidation reactor, the secondary components mentioned are obtained in the bottoms with the aqueous AA. The concentration of the secondary components (sum total of phthalic acid, terephthalic acid or isophthalic acid) is 0.024 to 0.030% by weight, based on the composition obtained in the quench tower bottoms.
The quench tower is frequently designed such that it can cool and condense the gaseous reaction mixture, but can also minimize the loss of acrylic acid in the tower tops. The quench tower is therefore typically equipped with distillation trays, for example bubble-cap trays or tunnel-cap or Thormann trays. Valve trays are also used.
If the throughput is to be increased significantly by means of such a quench tower, or the tops loss of the target product is to be reduced, it is frequently customary in the case of distillation trays to replace the trays with packing elements, especially structured packing elements.
It has been found here, however, that only a portion of the high boiler compounds is now discharged in the quench tower bottoms, and the other portion gets into the top of the column as an aerosol or desublimate, and leads to blockages therein and possibly in downstream apparatuses, such that the plant has to be shut down after only a short run time in order to conduct cleaning operations. In the column bottoms, the abovementioned secondary components are detected only in a concentration of <0.02% by weight based on the composition in the quench tower bottoms.